A urethane-based coating composition has been developed which finds particular use in construction and industrial applications. For example, bridge decking is susceptible to severe corrosion problems from winter road salting where the bridge deck is constructed from Portland cement concrete. The concrete is reinforced with steel reinforcing rods and is somewhat porous. When salt solution penetrates the concrete, it corrodes the steel, whose corrosion products, in turn, expand the concrete forcing it outward, spalling it from the bridge deck. The urethane-based composition is also used over underwater cement or concrete surfaces such as a pond liner or reservoir. This coating helps eliminate or reduce contamination and water seepage problems. Both examples illustrate eivronmentally rigorous usages, particularly requiring a specialized protection as disclosed in the present invention.
Traditionally, seal coats or surfacing compositions have been applied directly to concrete surfaces or structures to seal or protect them. One particular problem experienced in applying these coats, however, is the formation of bubbles in and under the coating. This problem is particularly severe with concrete containing a high percentage of entrained air. Before the mid-1960's concrete bridge decks, for example, contained very little air, usually less than 4%. Since then bridge decking concrete has been required to have about 6.5% entrained air to provide resistance to scaling. With this much air entrapped in the concrete, seal coats applied over it suffer severe air bubble formation problems. The bubbles are very difficult to remove and in most cases their removal destroys the integrity of the seal coat. On the other hand, if the bubbles are not removed the seal coat is considerably weakened at these points.
An additional problem which develops in the application of a seal coat over concrete is the presence of some foreign material such as mud, silt or grease which interferes with the adhesion of the seal coat to the concrete. This is a particular problem in situations where the concrete and surfacing coat are to be maintained underwater or in a situation, such as a roof top, which may be susceptible to intermittent wetting by rain. The presence of foreign matter such as mud, etc. gives rise to large surface-failure blisters or bubbles ranging from 6 inches to some 3 feet or larger in diameter.
U.S. Pat. No. 4,025,683 discloses a urethane based waterproofing coating composition which attempts to overcome the type of bubbling problem encountered in the first instance cited above; namely, air trapped in the concrete expanding under elevated temperature and rising to the surface upon application of the coating membrane. The invention of that patent seeks to address that problem by providing a primer coat consisting of a paint binder and a heat absorber which draw the entrapped air from the surface of the concrete and eliminate it prior to the application of the surface membrane.
The present invention helps solve not only that problem, entrapped air rising to the surface, but more particularly deals with adhesive failure after the surface membrane has cured due to the second type of bubbling failures, those caused by foreign matter interfering with the adhesion of the surface coating to the concrete. Moreover, the surface of the concrete normally contains small irregularities which trap air in the surface upon application of the surfacing composition. These also contribute to the failure of adhesion, independent of any temperature rise or fall. Both of these additional bubbling problems are overcome by the present invention, primarily because of the enhanced adhesive nature of the prime coat, versus the heat-induced air bubble expansion of the prior art.